Portable communication devices, such as cellular telephones, portable computers, personal digital assistants (PDAs), and the like, are configured to communicate over wireless networks. Such portable communication devices may enable communication over multiple networks, each of which has corresponding transmit and receive frequency bands within a composite broadband frequency range. Depending on design requirements, the frequency bands may have large spectrums and/or may be separated from one another by a significant range of frequencies. For example, the composite broadband frequency range may span from about 1700 MHz to about 2170 MHz, and may include multiple time division duplex (TDD) frequency bands of networks over which a communication device is able to transmit and receive radio frequency (RF) signals, such as band 1 (uplink 1920-1980 MHz; downlink 2110-2170 MHz), band 2 (uplink 1850-1910 MHz; downlink 1930-1990 MHz), band 3 (uplink 1710-1785 MHz; downlink 1805-1880 MHz), band 4 (uplink 1710-1755 MHz; downlink 2110-2155 MHz), and band 25 (uplink 1850-1915 MHz; downlink 1930-1995 MHz), for example. To provide filtering of the RF signals in a composite broadband frequency range requires a very large passband for transmitting and receiving the full range of frequencies. Very wide bandwidth band pass filters are therefore provided to accommodate the large passbands. A very wide bandwidth may be considered any bandwidth in excess about eight percent of a center frequency fcenter.
Various different types of band pass filters may be used in communication devices, including LC filters reliant on inductors and capacitors, and acoustic filters reliant on acoustic resonators. The acoustic resonators may include surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, for example, where the BAW resonators may include thin film bulk acoustic resonators (FBARs) and/or solidly mounted resonators (SMRs). Generally, LC filters are able to provide very wide bandwidths. However, LC filters do not provide sufficiently steep roll-off for corresponding passbands at the very wide bandwidths, with acceptably low insertion loss for efficient operation. Roll-off is a filter characteristic indicating how quickly the transition occurs between the filter passband and the filter stop band on either side of the passband, where the steeper the roll-off, the more efficient the transition. Generally, the steepness of the roll-off increases (i.e., thus improves) using higher order filters. However, the higher the order of an LC filter, in particular, the greater the insertion loss. However, acoustic filters are not able to accommodate sufficiently wide bandwidths, e.g., due to limited intrinsic acoustic coupling, to be used as very wide bandwidth filters.
Accordingly, there is a need for a band pass filter that combines a very wide passband with a steep roll-off, without unacceptable increases to insertion loss.
In addition, portable devices configured to communicate over multiple frequency bands, and thus multiple wireless networks, may include transmitters, receivers and corresponding band pass filters in multiplexers, connecting the receivers and transmitters to a common antenna, for sending and receiving signals over the multiple wireless networks. A multiplexer interfaces between the antenna and each of the networks to enable transmitting signals on different transmit (uplink) frequencies and receiving signals on different receive (downlink) frequencies. Conventional multiplexers providing band pass filters for different networks tend to be complicated in design, and to have relatively high insertion loss.
What is needed therefore is a filter that overcomes at least the drawbacks described above.